This application claims the priority of German patent application No. 100 52 636.5 filed Oct. 24, 2000, which is incorporated herein by reference.
The invention relates to a method for producing an ultrasound transducer of the type having a composite body that is comprised of plastic with a plurality of embedded piezoelectric or electrostrictive ceramic elements extending between the top side and the underside of the composite body, and electrodes that contact the ceramic elements on the top side and underside of the composite body.
A known ultrasound transducer of this type (U.S. Pat. No. 5,950,291), referred to as a composite acoustic transducer, has a plurality of ceramic elements comprising a piezoelectric or electrostrictive ceramic material, e.g., PZT, which are disposed in matrix fashion (1-3 composites). The ceramic elements are embedded in a rigid polymer layer, and form a composite body with this layer. The composite body is coated on its top side and underside with an electrode that contacts the ceramic elements extending between the top side and underside.
In the production of this ultrasound transducer, a ceramic body formed by an array of individual, columnar ceramic elements that protrude at a right angle from a ceramic base is placed into a casting mold, and the mold is filled to a specified level with a polymer that occupies the empty spaces between the ceramic elements. After the plastic has hardened, a solid plastic layer covers the ceramic base and surrounds the lower region of the ceramic elements. The partially-cast ceramic body is removed from the casting mold, rotated by 180xc2x0 and re-inserted into the mold, with the free ends of the ceramic elements being supported on the floor of the casting mold. The polymer is again poured into the casting mold to a certain level. After the plastic layer has hardened, the cast ceramic body is removed from the mold and the ceramic base is severed. The composite body formed in this manner is coated on its top side and underside with the electrodes.
The ceramic body having a plurality of ceramic elements that project from the ceramic base is obtained either through a casting process or the sawing of a ceramic block. In the latter case, the saw blade is inserted crosswise, and only so deep that the lower ceramic base remains. The casting method requires the ceramic elements to be conical for the removal of the ceramic body from the mold, so the ceramic elements cannot possess a constant cross-section over their length. The drawback of the sawing method lies in the high reject rate. The individual sawed ceramic elements shatter easily due to the brittleness of the ceramic material, thus rendering the entire ceramic body unusable.
It is the object of the invention to simplify and reduce the costs of the method for producing the ultrasound transducer described at the outset, thereby attaining a low reject quota, for lowering the costs of mass-producing the transducer.
In accordance with the invention, the above object generally is achieved by a method for producing an ultrasound transducer, the transducer having a composite body that comprises a plastic matrix with a plurality of embedded piezoelectric or electrostrictive ceramic elements extending between the top side and the underside of the composite body, and electrodes that contact the ceramic elements on the top side and underside of the composite body, with the method comprising the following method steps;
producing the ceramic elements as ceramic rods provided with a plastic jacket of a constant thickness,
conveying the jacketed ceramic rods to a container that is open on one side and is subjected to vibratory movements;
arranging the jacketed ceramic rods inside the container, while standing upright, by the vibratory movements of the container;
filling the container containing the jacketed ceramic rods with a plastic by bubbleless casting;
hardening the plastic to form a composite body and then removing the resulting composite body from the container;
grinding down at least one of a top side and an underside of the composite body until the ceramic rods possess a length stipulated by the working frequency of the ultrasound transducer; and
mounting the electrodes to the topside and underside of the composite body such that they contact all or only groups of the ceramic rods.
The method according to the invention has the advantage that the plastic-jacketed ceramic rods, whose cross-sectional profile can be round or polygonal and can be solid or hollow, can line up in matrix fashion when in the upright position due to simple vibratory movements, with the virtually constant thickness of the plastic jacket assuring a sufficiently constant spacing between the individual ceramic rods. After hardening, the plastic that fills the gaps between the rows of ceramic rods in the bubbleless casting, and is preferably a polymer such as resin or polyurethane, binds the jacketed ceramic rods securely to one another, thereby creating a composite body that either already possesses the desired shape or can be cut or sawed to the desired shape.
Practical embodiments of the method of the invention, with advantageous modifications and embodiments of the invention, likewise are disclosed.
In accordance with an advantageous embodiment of the invention, the plastic-jacketed ceramic rods are obtained as follows: Ceramic threads produced through spinning are cut into thread segments of the required length of the ceramic rods, plus some excess, and then polarized. They are then provided with a uniform plastic coating while moving in an immersion bath.
In accordance with an alternative embodiment of the invention, the ceramic threads can first be jacketed with the plastic layer of constant thickness in an immersion bath, and the thread segments forming the completely-jacketed ceramic rods can then be cut to the required length, plus some excess. In this instance, a high-temperature-proof plastic must be selected for the jacket, because the jacketed ceramic rods must still be polarized in a hot oil bath.
In accordance with an advantageous embodiment of the invention, the container exposed to the vibratory movements is covered with a perforated mask, whose holes have a slightly larger cross-section than the cross-section of the jacketed ceramic rods. A perforated mask of this type facilitates the vertical orientation and arrangement of the jacketed ceramic rods in the container. The perforated mask is removed before the jacketed ceramic rods that were aligned in the container are cast.
An ultrasound transducer produced in accordance with the method of the invention likewise is disclosed.
The invention is described in detail below using an exemplary embodiment illustrated in the drawings.